Decorative lamp



FIG. 1 is a perspective view of a decorative lamp showing my new design;

FIG. 2 is a left-side view thereof;

FIG. 3 is a right-side view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a front view thereof;

FIG. 6 is a back view thereof. 

The ornamental design for a decorative lamp, substantially as shown and described. 